How to write a Linux kernel module

Kernel modules are pieces of code that can be loaded and unloaded into the kernel upon demand. They extend the functionality of the kernel without the need to reboot the system.

## obtaining information
# list loaded modules
$ lsmod
# show module info
$ modinfo MODULENAME
# list dependencies
$ modprobe --show-depends MODULENAME

## automatic module load
# configure udev/systemd-modules to what modules to load at boot, see 'man modules-load.d'
$ vi {/etc,/run,/usr/lib}/modules-load.d/PROGRAM.conf
MODULENAME

## manual module load
# load by name
$ modprobe MODULENAME
# load by filename from '/lib/modules/$(uname -r)/'
$ insmod FILENAME [ARGS]
# unload module
$ modprobe -r MODULENAME
# same
$ rmmod MODULENAME

## passing parameters to module
# either from '/etc/modprobe.d'
$ vi /etc/modprobe.d/FILENAME.conf
options MODULENAME parametername=parametervalue
# or from kernel command line
MODULENAME.parametername=parametercontents

## blacklisting: prevent the kernel module from loading
# either from '/etc/modprobe.d'
$ vi /etc/modprobe.d/FILENAME.conf
blacklist MODULENAME
# or from kernel command line
modprobe.blacklist=modname1,modname2,modname3

from kernel modules@arch

You can write your own modules, see the linux kernel module programming guide.

# install build dependencies (kernel source)
$(deb) apt-get install build-essential linux-headers-$(uname -r)
$(el) yum install yum install gcc gcc-c++ make kernel-headers
$(arch) pacman -Syu base-devel linux-headers

# write a hello world module
$ vi hello.c
#include <linux/module.h> // all kernel modules
#include <linux/kernel.h> // KERN_EMERG, KERN_ALERT, KERN_CRIT, ... 
#include <linux/init.h>   // __init and __exit macros
MODULE_LICENSE("GPL");
MODULE_AUTHOR("You");
MODULE_DESCRIPTION("A Simple Hello World module");
static int __init hello_init(void) {
    printk(KERN_NOTICE "Hello world!n");
    return 0; // non-0 means init_module failed
}
static void __exit hello_cleanup(void) {
    printk(KERN_NOTICE "Cleaning up module.n");
}
module_init(hello_init);
module_exit(hello_cleanup);

$ Makefile
obj-m += hello.o
all:
    make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules
clean:
    make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean

# testing
$ make ; sudo insmod hello.ko
$ dmesg|grep -i hello
$ sudo rmmod hello.ko

from how to write your own linux kernel module

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